The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventor, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Electric vehicle (EV) relies on an electric motor, or multiple electric motors, for its propulsion. One of the key issues with respect to electric vehicle operation is achieving high system efficiency. Optimization of the EV system efficiency under all operating conditions has been among the top, if not the very top, endeavor of all engineering efforts devoted to the development of electric vehicle technologies. Inevitably, improvement of the electric motor efficiency is also a focus point of EV technology development.
Electric motors may be designed in various forms; however, each of the motors possesses operating points with higher efficiency and those with lower efficiency as well. When an electric motor is used for operation of various machine tools and equipment with fairly constant speed and torque, the system can be designed so the motor is operated at its highest efficiency for most of the time. However, when electric motors are used for electric vehicle propulsion, the situation is quite different from machine tools.
An electric vehicle needs to operate in a wide range of torque and speed to meet all performance requirements. For example, an electric vehicle needs to cruise in constant speed on leveled ground of city roads, in which the speed may range from low speed to high speed. The electric vehicle may also need to climb up slopes of various ramps and bridges, or even roads over to a hill. The electric vehicle may need to provide certain level of acceleration so it would not interrupt the normal traffic flow. The electric vehicle may even need to accommodate a fairly wide range of load variation, for example, in delivering vehicles or in buses. Therefore, it can be appreciated that electric motor operation in electric vehicles is quite different from that in machine tools.
While the electric motor operation needs to cover a wide range of torque and speed in electric vehicle propulsion and there is no way to fix the operation on the motor operating point with the highest motor efficiency, the motor design for electric vehicles faces a dilemma of providing the electric vehicle with desired high performance or providing the electric vehicle with an overall higher efficiency. When the electric motor needs to provide high performance for the electric vehicle, higher power and higher toque are needed to provide vehicle operation of high acceleration, high capability of slope climbing with desired, that is, higher, speeds while carrying higher loads. In this case, the motor may be operated in the high-efficiency point with these high-demand operations. However, when the electric motor is driven on leveled ground of city roads for most of the time, the efficiency becomes relatively low.
On the other hand, if the electric motor operation is to provide a higher overall efficiency over the electric vehicle operation, the motor needs to be designed with limitation where lower power and lower torque are provided. In this case, during the most operation of the electric vehicle over the leveled ground of city roads, the efficiency becomes higher. Yet, in such system, the electric vehicle can only have a lower level of acceleration, can only climb up slopes with a relatively low speed, and can only has a lower capability of starting over an inclined road of significant slope.